Abstract: In view of the special problems encountered in large caisson for the surge shaft construction in sand gravel overburden, a hydropower project is taken as an example. Considering large scale of the caisson, the poor uniformity of formation, the quantity of boulders and it was greatly effected by the formation property and construction factors, and the well wall being deviation and inclination, failure of the cutting edge and the well edge disengagement from formation during prophase construction which had seriously affected the construction safety, firstly, the problems of construction and their genesis were both analyzed. After then the change law of caisson wall earth pressure during construction is compared by an analytic method, the analysis results show that the earth pressure changed from static earth pressure to passive earth pressure when the deviation of the caisson appeared, and the well wall was not reliable in this case. Finally, the construction method for the caisson in the formation which contained boulders was compared by a numerical method during the sinking process, the analysis results show that adjusting the prophase sinking excavation method for the caisson to breaking boulders at first, then excavating soil, it can remarkally reduce the stress values of the caisson, effectively avoid the destruction of well wall and the cutting edge. Thus the improvement construction method for a large caisson of the surge shaft was proposed, which can be applied to same hydropower works under the special geological conditions.